1. Field of the Invention
This invention relates to the field of resistive elements, and more particularly, to a MOS voltage controlled linear resistor with superior dynamic range.
2. Art Background
Electrical circuits frequently require the use of resistive elements. Resistive elements, or resistors, can take any number of forms. One such resistor is the voltage-controlled resistor wherein the resistive value can be variably adjusted through the use of a control voltage. Voltage-controlled resistors find numerous applications, including use in PLL clock generators, adjustable amplifiers, and adjustable filters. A voltage-controlled resistor preferably provides a linear response over as wide a dynamic range as possible.
In a recent Transaction Brief published in the October 1990 "IEEE Transactions On Circuits And Systems," (Vol. 37, No. 10), three authors, G. Moon, M. E. Zaghloul, and R. W. Newcomb, disclosed an enhancement-mode MOS voltage-controlled linear resistor. In this disclosure, the authors describe a voltage-controlled linear resistor wherein a triode MOSFET is coupled in parallel to a diode connected MOSFET, and a control voltage is applied to the gate of the triode MOSFET. Although this voltage-controlled resistor provides certain advantageous characteristics, it suffers from a number of shortcomings. Principal among these is its limited dynamic range, or range of possible resistive values. In particular, this voltage-controlled resistor offers limited performance at low control voltages.
As will be described, the present invention provides for an MOS voltage-controlled linear resistor with considerable dynamic range. In particular, the present invention yields a dynamic range which is superior to the dynamic range of the voltage-controlled resistor disclosed by Moon, Zaghloul, and Newcomb. The present invention incorporates a first triode MOSFET, a second triode MOSFET, and a single diode connected MOSFET to yield a superior MOS voltage-controlled linear resistor.